The optimal dosage regimen required to obtain the maximum benefit from antiretroviral therapy with didanosine, zalcitabine, and stavudine is determined through the control of pharmacokinetic and pharmacodynamic sources of interpatient variability. The study seeks to determine the pharmacokinetic disposition of didanosine, zalcitabine and stavudine in the outpatient "real world" environment, evaluate intra and interpatient variability in blood concentrations of these agents and develop a basis for evaluating concentration-controlled antiretroviral regimens.